The general division of four-wheel drive systems is into two categories. The first category comprises part-time systems in which the transfer case does not include an inter-axle differential. The second category comprises full-time systems wherein a differential is located in the transfer case to permit the front and rear propeller shafts to rotate at different speeds when both the front and rear axles are being driven.
In a part-time four-wheel drive system conventional locking hubs may be used in connection with the front wheels. When the hubs are unlocked, the front wheels can rotate free of the front drive mechanism. In addition, the chain and sprocket assembly of the transfer case is disengaged to still the front drive mechanism to effectively obtain two-wheel drive operation. One limitation of a part-time system is that it does not include an inter-axle differential and operation in the four-wheel drive mode is not recommended for general highway use. Another limitation of this system is that in changing between two and four-wheel drive modes, the driver must perform the manual operations required to lock or unlock the wheel hubs and engage or disengage the chain and sprocket assembly to the front wheels.
As mentioned hereinabove, a full-time system has an inter-axle differential within the transfer case. One limitation of a full-time system of this general type is that the front drive mechanism is under full duty requirements and four-wheel drive is provided even when driving conditions may not warrant it. It is generally more efficient to operate the vehicle in two-wheel drive mode under normal driving conditions and change to four-wheel drive mode only when driving conditions become more demanding.
The preferred design concept and the subject of this application is a full-time system which permits operation in either two-wheel drive mode or four-wheel drive mode, and further allows for the transition between the modes to be performed automatically by a control system, and without need for substantial manual effort operations to perform the tasks. Important to this concept are improved means by which the front drive mechanism can be stilled when in two-wheel drive mode and made operational when in four-wheel drive mode.
Prior patents disclose various means for enabling the front wheel(s) to rotate free of the remainder of the front drive mechanism when in two-wheel drive mode. For example, the U.S. Pat. of Anderson No. 2,913,929 discloses a front axle design having complementary mechanisms for cooperatively disconnecting both of the front wheels from the drive train to permit two-wheel drive operation. These mechanisms are actuated in common by a movable lever in the passenger compartment of the vehicle. The movable lever is connected to complementary clutch collars which are shiftable between two-wheel drive and four-wheel drive positions on the right and left axle shafts. Likewise, the U.S. Pat. of Culverwell No. 2,770,150 discloses a front axle design for a four-wheel drive vehicle which includes a mechanism for simultaneously disconnecting the right and left axle shafts from the center differential. In this type of system, disconnection of the front wheels takes place by shifting complementary clutch collars located within the front differential housing. In the U.S. Pat. of Morgan No. 2,084,406 another disconnect system includes shiftable clutch members which are also contained within the front differential housing. The U.S. Pat. of Crispen No. 1,440,341 discloses still another disconnect system in which a shift fork controls movement of a clutch collar or sleeve to slidably connect or disconnect spindle sections.
Prior patents disclose various actuating means for shifting gears to obtain various modes of vehicle operation. For example, the U.S. Pat. of Rockwell et al No. 2,384,781 discloses a pneumatic power unit which actuates an arm which controls selection of the gear ratio in a differential. Likewise, the U.S. Pat. of Pringle No. 2,754,695 discloses an air-pressure actuated shift mechanism associated with the transfer case of a vehicle. In the U.S. Pat. of Hart et al No. 3,788,166 a hydraulically operated disc clutch of a differential assembly is controlled by a control system having both manual and automatic modes.
Other control systems of possible interest are disclosed by the U.S. Pat. Nos. of Paxton 1,284,759; Middlesworth et al 3,522,861; Hawk 3,058,558; Beard et al 3,552,516; Kaiser 3,283,298, Johnson Jr. et al 3,354,741; Hammond 3,256,750; Ordorica et al 3,295,625; Sampietro et al 3,221,574 and Jeffers 3,871,249.